Field of the Invention
The present invention relates to an ultrasonic motor, a drive control system and an optical apparatus that use the ultrasonic motor, and further, to a vibrator to be used in the ultrasonic motor.
Description of the Related Art
A vibration-type (vibrational wave) actuator includes a vibrator configured to excite vibration in an elastic body having an annular shape, an elliptical shape, a bar shape, or the like, which is joined to an electromechanical energy converting element, e.g., a piezoelectric element, by applying an electric signal, e.g., alternating voltage to the electromechanical energy converting element. The vibration-type actuator is used, for example, as an ultrasonic motor configured to relatively move an elastic body (moving member) that is brought into pressure-contact with the vibrator and the vibrator (static member) through use of the drive force of the vibration excited in the vibrator.
Now, an overview of the structure and drive principle of an annular ultrasonic motor that is a typical usage form of the vibration-type actuator is described. In the following description, the term “annular” is intended to mean that an annular article or element can be schematically regarded as a configuration in which a disc having a predetermined thickness includes a circular through hole concentrically. In this case, the dimension of the disc-shaped article or element corresponding to the thickness of the disc is referred to as “thickness” of the article or element, and respective surfaces of the annular article or element corresponding to both surfaces of the disc that hold the thickness of the disc are individually or generically referred to as “surfaces” of the article or element.
The annular ultrasonic motor includes an annular vibrator and an annular moving member that is brought into pressure-contact with the vibrator. The moving member is formed of an elastic body, and a metal is generally used as a material for the moving member. The vibrator includes an annular vibrating plate and an annular piezoelectric element arranged on one surface of the vibrating plate. The vibrating plate is formed of an elastic body, and a metal is generally used as a material for the vibrating plate. The piezoelectric element includes, on one surface of an annular piezoelectric ceramics, an electrode divided into a plurality of regions along the circumferential direction of the annular ring and one common electrode on the other surface thereof. A lead zirconate titanate-based material is generally used as a material for the piezoelectric ceramics.
The electrode divided into a plurality of regions includes two regions forming drive phase electrodes, at least one region forming a detection phase electrode, and a region forming a non-drive phase electrode, which is arranged as necessary. Wiring configured to input electric power for applying an electric field to a corresponding region of the annular piezoelectric ceramics that is brought into contact with each drive phase electrode is arranged in each drive phase electrode, and the wiring is connected to a power source unit.
A circle that passes through an arbitrary position on the surface of the annular piezoelectric element and shares the center with the annular ring is assumed, and the length of one arc obtained by dividing the circumference of the circle by n (n is a natural number) is represented by λ, and the circumferential length of the circle is represented by nλ. A region of the piezoelectric ceramics corresponding to the region forming each drive phase electrode is subjected to polarization treatment in advance by applying an electric field to the piezoelectric ceramics in a thickness direction thereof alternately in an opposite direction at a pitch of λ/2 along the circumferential direction. Therefore, when an electric field in the same direction is applied to the piezoelectric ceramics in the thickness direction with respect to all the regions, the expansion and contraction polarity of the piezoelectric ceramics in the regions is reversed alternately at a pitch of λ/2. The two regions forming the respective drive phase electrodes are arranged at a distance of an odd multiple of λ/4 in the circumferential direction. In general, two regions (spacing regions) that separate the two drive phase electrodes from each other include non-drive phase electrodes that are short-circuited to a common electrode so that piezoelectric vibration is not caused spontaneously, with the result that an electric field is not applied to the piezoelectric ceramics in those regions. In general, a detection phase electrode is arranged in the spacing region as described later.
When an alternating voltage is applied to only one of the drive phase electrodes of such an ultrasonic motor, a first standing wave having a wavelength λ is generated over the entire circumference of the vibrator. When an alternating voltage is applied to only the other drive phase electrode, a second standing wave is generated similarly, but the position of the wave is rotated and moved by λ/4 in the circumferential direction with respect to the first standing wave. Meanwhile, when alternating voltages, which have the same frequency and a temporal phase difference of π/2, are applied to the respective drive phase electrodes, a propagating wave (wave number along the annular ring: n and wavelength: λ) of bending vibration (vibration having an amplitude perpendicular to the surface of the vibrator), which propagates in the circumferential direction over the entire circumference, is generated in the vibrator as a result of the synthesis of both the standing waves.
When the propagating wave of the bending vibration (hereinafter sometimes simply referred to as “bending vibration wave”) is generated, each point on the surface of the vibrating plate forming the vibrator undergoes an elliptical motion. Therefore, the moving member that is brought into contact with the surface rotates due to friction force (drive force) in the circumferential direction from the vibrating plate. The rotation direction can be reversed by switching, between positive and negative, a phase difference π/2 of the alternating voltage applied to each drive phase electrode. Further, the rotation speed can be controlled by changing the frequency and amplitude of the alternating voltage applied to each drive phase electrode.
The generated bending vibration wave can be detected with the detection phase electrode arranged in the spacing region. That is, the distortion of deformation (vibration) generated in the piezoelectric ceramics brought into contact with the detection phase electrode is converted into an electric signal in accordance with the magnitude of the distortion and output to a drive circuit through the detection phase electrode.
When an alternating voltage is applied to the ultrasonic motor at a frequency higher than a resonant frequency, the ultrasonic motor starts a rotation operation. When the frequency is brought close to the resonant frequency, the rotation is accelerated to reach a highest rotation speed at the resonant frequency. Thus, the ultrasonic motor is generally driven at a desired rotation speed by sweeping the frequency from a frequency region higher than the resonant frequency to the resonant frequency.
Meanwhile, a lead zirconate titanate-based material to be used in the piezoelectric ceramics contains a large amount of lead in an A-site of an ABO3 perovskite type metal oxide. Accordingly, an effect of a lead component on environments has been seen as a problem. In order to deal with this problem, piezoelectric ceramics using a perovskite type metal oxide that does not contain lead (lead content is less than 1,000 ppm) has been proposed.
In Japanese Patent No. 5217997, there is disclosed piezoelectric ceramics in which a part of an A-site of barium titanate is substituted with calcium (Ca), and an ultrasonic motor using the piezoelectric ceramics. In Japanese Patent Application Laid-Open No. 2009-227525, there is disclosed piezoelectric ceramics in which a part of sodium niobate is substituted with barium titanate.
However, the piezoelectric constant of each of the piezoelectric ceramics disclosed in Japanese Patent No. 5217997 and Japanese Patent Application Laid-Open No. 2009-227525 was about a half of that of a lead zirconate titanate-based material. Even when an ultrasonic motor was manufactured through use of a vibrator developed on the premise of using the lead zirconate titanate-based material, a sufficient rotation speed of the ultrasonic motor was not able to be obtained.